In a first, public, mode of a display device that is switchable between a public and private display mode, the device commonly behaves as a standard display. A single image is displayed by the device to as wide a viewing angle range as possible, with optimum brightness, image contrast and resolution for all viewers. In the second, private mode, the main image is discernible only from within a reduced range of viewing angles, usually centred on the normal to the display surface. Viewers regarding the display from outside this reduced angular range will perceive either a second, masking image which obscures the main image, or a main image so degraded as to render it unintelligible.
This concept can be applied to many devices where a user may benefit from the option of a privacy function on their normally wide-view display, for use in certain public situations where privacy is desirable. Examples of such devices include mobile phones, Personal Digital Assistants (PDAs), laptop computers, desktop monitors, Automatic Teller Machines (ATMs) and Electronic Point of Sale (EPOS) equipment. Such devices can also be beneficial in situations where it is distracting and therefore unsafe for certain viewers (for example drivers or those operating heavy machinery) to be able to see certain images at certain times, for example an in car television screen while the car is in motion.
Several methods exist for adding a light controlling apparatus to a naturally wide-viewing range display.
One such structure for controlling the direction of light is a ‘louvred’ film. The film consists of alternating transparent and opaque layers in an arrangement similar to a Venetian blind. Like a Venetian blind, it allows light to pass through it when the light is travelling in a direction nearly parallel to the layers, but absorbs light travelling at large angles to the plane of the layers. These layers may be perpendicular to the surface of the film or at some other angle. Methods for the production of such films are described in a USRE27617 (F. O. Olsen; 3M 1973), U.S. Pat. No. 4,766,023 (S.-L. Lu, 3M 1988), and U.S. Pat. No. 4,764,410 (R. F. Grzywinski; 3M 1988).
Other methods exist for making films with similar properties to the louvred film. These are described, for example, in U.S. Pat. No. 5,147,716 (P. A. Bellus; 3M 1992), and U.S. Pat. No. 5,528,319 (R. R. Austin; Photran Corp. 1996).
Louvre films may be placed either in front of a display panel or between a transmissive display and its backlight to restrict the range of angles from which the display can be viewed. In other words, they make a display “private”.
The principal limitation of such films is that they require mechanical manipulation, i.e. removal of the film, to change the display between the public and private viewing modes.
A method for switching from public to private mode with no moving parts is to mount a light control film behind the display panel, and to place a diffuser which can be electronically switched on and off between the light control film and the panel. When the diffuser is inactive, the light control film restricts the range of viewing angles and the display is in private mode. When the diffuser is switched on, it causes light travelling at a wide range of angles to pass through the panel and the display is in public mode. It is also possible to mount the light control film in front of the panel and place the switchable diffuser in front of the light control film to achieve the same effect.
Switchable privacy devices of these types are described in U.S. Pat. No. 5,831,698 (S. W. Depp; IBM 1998), U.S. Pat. No. 6,211,930 (W. Sautter; NCR Corp. 2001) and U.S. Pat. No. 5,877,829 (M. Okamoto; Sharp K. K. 2001). They share the disadvantage that the light control film always absorbs a significant fraction of the light incident upon it, whether the display is in public or private mode. The display is therefore inefficient in its use of light. Since the diffuser spreads light through a wide range of angles in the public mode, these displays are also dimmer in public than in private mode, unless the backlight is made brighter to compensate.
Another disadvantage relates to the power consumption of these devices. In the public mode of operation, the diffuser is switched off. This often means that voltage is applied to a switchable polymer-dispersed liquid crystal diffuser. More power is therefore consumed in the public mode than in the private mode. This is a disadvantage for displays which are used for most of the time in the public mode.
In GB2413394 (Sharp), a switchable privacy device is constructed by adding one or more extra liquid crystal layers and polarisers to a display panel. The intrinsic viewing angle dependence of these extra elements can be changed by switching the liquid crystal electrically in the well-known way. Devices utilising this technology include the Sharp Sh851i and Sh902i mobile phones.
WO06132384A1 (Sharp, 2005) discloses the use of an extra liquid crystal layer located between the existing polarisers of a liquid crystal display (LCD) panel. In this location, the extra switch cell can modify the greyscale curves for off axis light. This provides a higher level of privacy for images than the techniques disclosed in GB2413394.
GB2439961 (Sharp) discloses the use of a switchable privacy device constructed by adding an extra cholesteric layer and circular polarisers to a display panel. The cholesteric layer can be switched between a public (wide view) mode and a private (narrow view) mode that can provide 360° azimuthal privacy.
The above methods all suffer the disadvantage that they require the addition of extra apparatus to the display to provide the functionality of electrically switching the viewing angle range. This adds cost, and particularly bulk to the display, which is very undesirable, particularly in mobile display applications such as mobile phones and laptop computers.
Methods to control the viewing angle properties of an LCD by switching the single liquid crystal layer of the display between two different configurations, both of which are capable of displaying a high quality image to the on-axis viewer are described in US20070040780A1 and GB 0721255.8. These devices provide the switchable privacy function without the need for added display thickness, but require complex pixel electrode designs and other manufacturing modifications to a standard display.
One example of a display device with privacy mode capability with no added display hardware complexity is the Sharp Sh702iS mobile phone. This uses a manipulation of the image data displayed on the phone's LCD, in conjunction with the angular data-luminance properties inherent to the liquid crystal mode used in the display, to produce a private mode in which the displayed information is unintelligible to viewers observing the display from an off-centre position. However, the quality of the image displayed to the legitimate, on-axis viewer in the private mode is severely degraded.
A similar scheme to that used on the Sh702iS phone, but which manipulates the image data in a manner dependent on a second, masking, image, and therefore causes that masking image to be perceived by the off-axis viewer when the modified image is displayed, is given in GB2428152A1. However, the image processing methods described in the above document rely on the data-value to luminance curve being strongly non-linear for off-axis viewing directions, which is not the case for modern LC display modes such as “Advanced Super View” (ASV) (IDW'02 Digest, pp 203-206) or Polymer Stabilised Alignment (PSA) (SID'04 Digest, pp 1200-1203). As a result, the privacy effect is not sufficiently strong in these displays in some circumstances to disguise the main image.
It is therefore desirable to provide a high quality LCD display which has public and private mode capability, in which no modification to the LC layer or pixel electrode geometry is required from a standard display, has a substantially unaltered display performance (brightness, contrast resolution etc) in the public mode, and in the private mode has a strong privacy effect with minimal degradation to the on-axis image quality.